Activation of actomyosin ATPase and tension development in smooth muscle is believed to be myosin-mediated via Ca2+ -calmodulin dependent phosphorylation of myosin light chain. However, existence of a thin filament based regulation, which complements the regulation via phosphorylation, is supported by biochemical studies. Tropomyosin, a protein located on the thin filament, potentiates the actin-activated ATP hydrolysis by myosin. Recent biochemical studies showed that the actin binding protein, caldesmon, inhibits the tropomyosin-enhanced actomyosin ATPase. In the presence of Ca2+, calmodulin binds to caldesmon and the caldesmon induced inhibition of the tropomyosin-enhanced actomyosin ATPase is reversed. Two of the major questions that pertain to the understanding of the roles of interactions of thin filament- associated proteins on the actomyosin ATPase and contraction in smooth muscle are: (a) How does caldesmon inhibit n potentiation of actin-activated ATP hydrolysis? and (b) How does Ca2+- calmodulin release the caldesmon induced inhibition? These two broad basic questions will be answered by experiments to obtain specific information on the interactions of thin-filament associated proteins on the actin and their effects on the binding of myosin heads to actin and ATP hydrolysis. Thin filament- associated proteins from medial smooth muscle of swine pulmonary arteries will be isolated. Actin filament will be reconstituted with other thin-filament proteins, tropomyosin, caldesmon and calmodulin, to test their roles in the regulation of actin- activated ATP hydrolysis by purified phosphorylated myosin, heavy meromyosin or subfragment-1. Data from these experiments will provide us with the basic information for understanding the protein-protein interactions on the thin filament and the actin- based regulation of actomyosin ATPase and the associated development and maintenance of force and relaxation in smooth muscle.